Change pru_abort() return type to the type of void and make pru_abort()

Change pru_abort() return type to the type of void and make pru_abort()
optional.

We have no interest on pru_abort() return value. We call it only from
soabort() which is dummy pru_abort() wrapper and has no return value.

Only the connection oriented sockets need to implement (*pru_abort)()
handler. Such sockets are tcp(4) and unix(4) sockets, so remove existing
code for all others, it doesn't called.

ok guenther@